Window to the Past

In the original version of this story, the paragraph that states, “This photon-absorbing barrier of gas creates a break in the light spectrum, Oesch said.” However, this information should be attributed to Garth Illingworth.

Also, in the original version of this story, the paragraph that states, “A team of researchers, including Illingworth, the project’s principal investigator, Pascal Oesch, and UCSC Hubble Fellow and Lick Observatory programmer Dan Magee,” should instead read, “A team of researchers including Illingworth — the project’s principal investigator — UCSC Hubble Fellow Dr. Pascal Oesch, and Lick Observatory programmer Dan Magee…”

Lastly, the sentence that read, “The team, named Magee, designed computer programs with meticulous accuracy,” should instead read “The team, namely Magee, designed computer programs with meticulous accuracy.” This post was updated on Oct. 28 to reflect this change.

Time machines exist.

At least, they do in the form of the eXtreme Deep Field (XDF), a recent deep space image put together by a team of UC Santa Cruz researchers that looks 13.2 billion years back in cosmic history.

The XDF, released on Sept. 25, covers a small patch of the southern sky on the constellation Fornax. The image displays 5,500 galaxies, some of which were formed as early as 450 million years after the Big Bang. Given that the Big Bang occurred 13.7 billion years ago, this duration of time constitutes no more than a fraction of a second to the time span of the universe. XDF is the first deep space image published in which such early galaxies are visible.

“XDF is a ‘Rosetta Stone’ for how galaxies formed and grew,” said Garth Illingworth, a UCSC astrophysics professor. “The history of galaxies is laid out in this remarkable, incredibly deep image as it looks back through 96 percent of all time.”

A team of researchers including Illingworth — the project’s principal investigator — UCSC Hubble Fellow Dr. Pascal Oesch, and Lick Observatory programmer Dan Magee, combined thousands of exposures taken by the Hubble Space Telescope to create the XDF.

The project involved a process of analyzing and compiling the exposures for XDF, which began after approval for the project was granted by NASA in 2009. The team, namely Magee, designed computer programs with meticulous accuracy.

Oesch said the faintest galaxies visible in XDF are hard to see because they are moving. The universe is rapidly expanding, and as galaxies move, light waves that they emit lengthen relative to the points in space from which they move away. As they lengthen, they are shifted towards the red end of the light spectrum in a phenomenon called “redshift.”

Oesch also said the earliest galaxies visible on XDF move away from our galaxy at tens of thousands of kilometers every second. When a light source moves at such high speeds, the light emitted demonstrates redshift in its extremes. The light that the newly visible XDF galaxies emit have been redshifted to the infrared zone of the light spectrum.

In addition, these early galaxies are surrounded by neutral gas, a remnant of the Big Bang. Today the atoms that comprise the gas that fills the space between galaxies are ionized, Oesch said. Ionization occurs when an atom loses or gains electrons, negatively charged atomic subparticles. This causes an atom to become positively or negatively charged.

In the early millennia of the universe, all gas in space was neutral, Oesch said. Ionization occurred when the first galaxies began to emit light. Light is comprised of photons, and when photons from the first galaxies collided with the neutral atoms surrounding them, they interfered with the atoms’ electrons and ionized them. However, neutral hydrogen persists around the earliest galaxies.

“As a result of this, the light from the early galaxies cannot freely travel toward us,” Oesch said. “But it continuously hits neutral hydrogen along its way, causing photons to be absorbed.”

This photon-absorbing barrier of gas creates a break in the light spectrum, Illingworth said. This break occurs roughly 800 million years after the Big Bang, and can be used as a datum point to gauge the position of galaxies visible in the XDF.

The redshifted galaxies that formed before this point in time were invisible in earlier deep space images.

“We cannot see the first galaxies in optical light, and we needed the near-infrared camera on Hubble to discover such sources,” Oesch said.

Wide Field Camera 3 (WFC3), the “near-infrared” camera to which Oesch refers, can detect the infrared light beyond this break in the light spectrum. Oesch said that because light waves from these galaxies take billions of years to reach us, their appearance on XDF is as it was billions of years before present.

According to NASA, the Hubble Space Telescope took 2 million seconds worth of exposures of this tiny patch of sky between 2002 and 2012. The image itself is a mosaic of thousands of exposures. Researchers combined the exposures taken between 2003 and 2004 to create the Hubble Ultra Deep Field image, XDF’s predecessor, and XDF added to these with exposures taken by WFC3.

“Our astronomical images are only black and white pictures. In order to reveal colors of galaxies, we take different images at different wavelengths. For the XDF image, we combined more than 2,000 images including optical and near-infrared light to create this beautiful color image,” said Oesch, who analyzed the original exposures.

Despite the enormous effort contributed to the project by Illingworth, Oesch, Magee and their colleagues, XDF has not yet reached as far back in space and time as future images will.

“We can only see the tip of the iceberg, essentially,” Oesch said. “We know that there are so many more galaxies that are fainter than what we can see now with Hubble.”

The Hubble Space Telescope has reached its technological limit 22 years after its initial launch. Oesch said that many more galaxies will be identified with the completion of the James Webb Space Telescope (JWST) in 2018.

Half-completed, astronomers can as of yet only hypothesize JWST’s future contributions to science. According to NASA, the area covered by XDF will be among its first targets, and images yielded by JWST will show the earliest galaxies at their origins.

Illingworth is highly involved with the creation and launch of the new $8 billion telescope.

“We’re going to put JWST way beyond the moon,” Illingworth said. “So it’s big, it’s complicated, it’s very expensive, and we’re going to throw it out into space. It better work.”

To learn more about XDF, view more images, and learn more about the research team, visit the XDF website at

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